How do you know if it's preterm labor or BH?

I'm having constant period like cramping that shoots around and down my thighs and my stomach is almost always right. At my check up my doctor said to drink more water which I have been doing but it's constant pain. I can't time them or anything because it doesn't go away. How do I know if it's serious or just my body doing its thing to get ready?? I'm 35 weeks.